152 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Andesites of the quicksilver belt. — Andcslte is abundant near Clear Lake, and 

 thence southward. This region was studied chiefly for its bearing upon 

 the geology of the Sulphur Bank, and no attempt was made to investigate 

 the separate varieties of andesite minutely. Besides the information to be 

 derived from specimens, however, it is certain that the andesite of this 

 region was ejected at two distinct periods, of which one preceded the 

 Pliocene Cache Lake beds. This earlier andesite is largely represented 

 in the conglomerates or uncompacted, pebbly beds of Cache Lake. The 

 later andesitic eruption which forms the mass of Mt. Konocti closed the 

 Cache Lake period very late in the Pliocene, but preceded the basalt 

 eruptions by an unknown though considerable interval. The older ande- 

 site, represented by Chalk Mountain and by the pebbles in the Cache 

 Luke beds, is a rather dense, bluish-gray rock, somewhat altered in most 

 cases. Under the microscope it is found to be composed of pyroxene and 

 feldspar crystals embedded in a groundmass consisting of feldspar niicro- 

 lites and magnetite. The feldspar of the groundmass is all microlitic and 

 the structure is similar to that of the "felted" groundmass so common in 

 pyroxene-andesites, except that it is irnusually coarse. The pyroxene is 

 mostly rhombic, but is very light colored and shows no dichroism. Pre- 

 cisely similar to this andesite is one from the northern end of Thurston 

 Lake. This occurrence appeared from field examination to be older than 

 the asperites b}' which it is accompanied, while from its similarity under the 

 microscope to the rock of Chalk Mountain it seems altogether probable that 

 it is of the sama age as the latter. These rocks contain no hornblende. 



The younger andesites of Clear Lake are referable to the group of 

 asperites discussed above. Macroscopicall}^ they are dark-gray or tawny, 

 soft, trachytic rocks, often showing lamination more or less distinctly. In 

 fact these lavas are entirely indistinguishable from some of the andesites of 

 Steamboat Springs. They are pyroxene rocks, and not a single slide shows 

 a particle of amphibole. In one slide, however, are a few minute patches 

 of opacite with iri-egular outlines, which possiblv, Ijut by no means cer- 

 tainly, replace hornblendes. On the other hand, mica is a frequent though 



which hornblende was the prevalent liisilicate. In the western United States the asperites are usually 

 micaceons and dense hornhleude-andesitea are abundant. The latter are generall.v distinct from the 

 Uensc, pyroxenic rocks. Giimbel divides his types at 57 per cent, of silica. 



